Astronomers are coming to believe that some of the solar system's moons might be more likely places to search for life than Mars.

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One of Jupiter's four large moons, Europa, is the leading candidate. Robert Pappalardo, a planetary scientist at JPL, considers it to be "the place we should be exploring now that we have a concept mission we think is the right one to get there for an affordable cost."

"Europa," he says, "is the most promising in terms of habitability because of its relatively thin ice shelf and [subsurface] ocean ... And we know there are oxidants on the surface of Europa." Those oxidants take the form of reddish deposits around the many fractures and fissures that lace the icy surface of the satellite. Red, orange and yellow colors are often a tell-tale sign of organic molecules.

JPL and the Applied Physics Laboratory at Johns Hopkins University have developed a Europa exploration project called "Clipper." Like Cassini, Clipper would consist of a spacecraft would conduct numerous close flybys of Europa while orbiting and a lander similar to the Huygens probe that sat down on Titan.

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"That way we can get effectively global coverage of Europa by doing many many flybys," said Pappalardo. "And that can do outstanding science — not quite as good as an orbiter, but not that bad — for half the cost." Pappalardo is all for searching for signs of past life on Mars. But, he says, while life may have existed on Mars billions of years ago it might still be thriving on Europa.

If all went well, Clipper would be launched by 2021. It would take three to six years to reach Europa. Unfortunately, not all is going well since NASA has not allocated funds for the mission. At the same time, however, NASA is sending another rover to Mars in 2020, to continue the search for life on that planet. All may not be lost, however. NASA has been invited by the European Space Agency to participate in a program called "Jupiter Icy Moon Explorer." This would involve a spacecraft that would arrive at Jupiter in 2030.

Other prime candidates among the solar system's moons for the search for life include Enceladus. "If Europa is the best place in the solar system after Earth to host life," says Amanda Hendrix, a senior scientist at the Planetary Science Institute, "Enceladus is right up there as well." Like Europa, it has subsurface seas, or at least lakes, and the tidal flexing that powers its geysers also provides heat that keeps the water liquid...and might also create a friendly environment for life. A third candidate is Titan. It's atmosphere and surface are filled with organics and several Cassini scientists have recently speculated that some of these might be the product of living processes.